This invention relates to processes for the preparation of ar-substituted alkylbenzenes and to a process for the preferential aliphatic halogenation of one isomer in an isomeric mixture of ar-substituted alkylbenzenes.
It is known in the art that 2,4-dihalo-1-alkylbenzenes can be isomerized to 3,5-dihalo-1-alkylbenzenes in the presence of a catalyst containing aluminum and bromine. See, for example, U.S. Pat. No. 3,553,274.
It is also known in the art that aliphatic (side chain) halogenation of, among other isomers, the ar,ar-dihaloethylbenzenes can be catalyzed by phosphorus trihalide or phosphorus pentahalide. Such reactions are typically conducted in the liquid phase at a temperature of between about 40.degree. C and about 80.degree. C. U.S. Pat. No. 2,265,312 teaches that the above-mentioned catalysts are useful in carrying out such aliphatic halogenation reactions.
In U.S. Pat. No. 2,432,737, a method is taught for the production of a mixture of ring isomers of dichlorostyrene. Both .alpha.-chlorination and .beta.-chlorination of the side chains of a mixture of ethyldichlorobenzene isomers result under the conditions of the process which are the bubbling of liquid chlorine through the mixture of isomers at a temperature of about 40.degree. C to 80.degree. C in the presence of a catalyst such as PCl.sub.3.
In U.S. Pat. No. 2,193,823, a method is taught for selective .alpha.-chlorination of an ethyl side chain of an ar-halo-ar-ethylbenzene. For example, this patent describes chlorinating the .alpha.-carbon of ethylpentachlorobenzene by contacting the same in the liquid phase with chlorine gas in the presence of the light of an ordinary electric light bulb to produce .alpha.-chloroethylpentachlorobenzene.
Heretofore, the preferential aliphatic halogenation of one isomer in a mixture of ar-substituted alkylbenzenes has not been disclosed.